Process for the production of shaped parts of plasticized polyethylene



United States Patent "ice 3,134,830 EROCESS FUR THE PRODUCTION OF SHAPEDPARTS OF PLASTICIZED PULYETHYLENE Otto Roelen, fiberhausen-Holten,Germany, assignor to Ruhrchernie Alttiengesellschaft, Oberhausen-Holteu,

Germany No Drawing. Filed Nov. 4, 1957, Ser. No. 694,038

Claims priority, application Germany Nov. 5, 1956 3 Claims. (Cl.2641t2d) It has already been suggested to produce shaped parts ofpolyethylene by compressing finely divided polyethylene, especiallypolyethylene powder, having molecular weights above 500,000, e.g. above1,000,000 in several pressure stages; the compressions in the secondpressure stage and, if necessary or desired, in the succeeding pressurestages is carried out with simultaneous heating until the material isplasticized.

Thus, blanks, having larger dimensions than those of the finished partsare first produced by mechanical pressure and preferably at roomtemperature. These blanks are subsequently converted into sintere'd andplasticized polyethylene by the simultaneous use of heat and ofmechanical pressure.

Good results have been obtained by this working method with regard tothe quality of the shaped parts produced. Also, the process could becarried out in a simple manner with the devices known in the art. Theproduction of larger shaped parts, however, was limited by the fact thatthe cold-pressing operation first required very high pressures on thewhole surface of the blanks to be finished. The cold-pressing operationrequires substantially higher pressures than the succeeding pressurestages effected with the material heated prior to or during thecompression.

It has been found that these difliculties can be avoided if the processfor the production of shaped parts of plasticized polyethylene mixed, ifdesired, with other polymers, is carried out in at least two compressionstages. In the first of these stages shape-retaining blanks of finelydivided material are produced with no increase in temperature which aresubsequently plasticized in a second compression stage after having beenheated or while being heated; this is done in such a manner that two ormore blanks resulting from the first pressure stage are placed togetherin face-toface position in the second pressure stage, so that they forman integral shaped part or body after having undergone the treatmenteffected in the second pressure stage.

It has surprisingly been found in this process that the seams betweenthe individual cold-pressed blanks are frequently no longer visible inthe space after sintering. Above all, these seams have the sameproperties with regard to strength as those of the surrounding portions.The important advantage of the process of the invention resides in thefact that it becomes possible to produce substantially larger shapedparts with the same hydraulic press available.

Thus, for example, slabs of 20 x 20 centimeters and 3 cm. thickness mayfirst to be produced by cold-pressing. These slabs may be placed side byside into a die of 1 x 2 meters. If at least one additional compressionis now applied while heating, one single sheet of 1 x 2 meters and 3 cm.thickness is obtained. Seams are no longer recognizable in the finishedsheet. According to another embodiment of the process of the invention,heating of the individual slab is not effected in the second compressionstage but previously as, for example, is an oven under a protective gasatmosphere.

It is also possible by the process of the invention, for example, tostack 5 slabs each of cm. thickness and press them into a block of about50 cm. thickness. The production of such a block from a single piecewould require excessively long heating periods due to the poor 3,134,830Patented May 26, 1964 heat conductivity of polyethylene so that theproduction would become uneconomical. Thus, the process of the inventionpermits the production of blocks of any thickness desired.

In practice, best results are obtained when the pressure applied in thefirst stage of the process is at least 5 times that of the pressure usedin the second stage. According to a specific embodiment of the process,the blanks obtained from the first stage consist of prisms having 6surfaces positioned at right angles .with respect to one another, or ofdisks of circular or other form with par allel surfaces. The prisms maybe arranged side by side in one layer, or the prisms or disks may beplaced one upon another. It is advantageous in many cases to placeprisms side by side and in layers.

While in general blanks having the same chemical composition and thesame physical properties will be used for the second and, as the casemay be, the succeeding compression stages, the process becomesparticularly useful by the fact that blanks having different propertiescan be combined to form an integral shaped part; in that case, it ispreferable to operate so that the difierent blanks are arranged inregular order as, for example, in layers.

Thus, it is possible to combine into one piece coldpressed pieces ofdifferent qualities, e.g. of dilferent colors, different molecularweights, etc., thereby producing laminates having particular coloreffects and particular properties with regard to strength.

It is favorable in special cases to incorporate other polymers into thepolyethylene. The process of the invention is also applicable to theproduction of shaped parts of plasticized polypropylene.

The process of the invention is primarily applicable to high molecularweight polyethylene, i.e., for example, polyethylenes having molecularweights in excess of 100,- 000 and preferably above 500,000.Polyethylene of this type is known as high-density polyethylene and isproduced by the polymerization of ethylene at pressures below about 100kg./ sq. cm. and temperatures up to about 100 C. with the use ofcatalysts consisting of mixtures of organometallic compounds, especiallyaluminum alkyl compounds, with compounds of metals of the 4th to 6thsubgroups of the periodic table, particularly with titanium compoundsas, for example, titanium tetrachloride (see Angewandte Chemie, vol. 67,541-547 (1955)).

The molecular weights stated above have been determinedviscosimetrically.

The process according to the invention will now be more fully explainedin a number of examples, but it should be understood that these aregiven by way of illustration and not of limitation and that many changesin the details can be made without departing from the spirit of theinvention.

Example 1 The polymerization of ethylene was effected in a glass vesselof about 5 liters capacity and equipped with an agitator, into whichwere filled 2 liters of a C -C hydrocarbon fraction obtained by carbonmonoxide hydrogenation. This hydrocarbon fraction had been prepared byhydrogenation, eifected at 250 C., subsequent refining with sulfuricacid, and intensive drying. After flushing of the reaction vessel withethylene gas and heating to about 50 C., the catalyst solution was addedwhile agitating and passing ethylene through the vessel. The catalystsolution had been prepared by mixing together 100 cc. of the same C Cfraction, 1.08 grams of diethyl aluminum monochloride and 0.42 gram oftitanium tetrachlo ride, and vigorously shaking the mixture for about 30minutes. Upon addition to the catalyst solution, the reactiontemperature was adjusted to about C. Atmospheric pressure prevailedthroughout the reaction as glass equipment cannot be subjected toelevated pressures.

After 12 hours of reaction, 472 grams of polyethylene had formed.

The mixture was filtered and the filter residue was treated with timesits quantity of 1% aqueous sodium hydroxyide solution in a stirringflask. The mixture was then heated to the boiling point, therebydistilling oil the residue of the hydrocarbon fraction together with thewater vapor.

The residue from the distillation was finally washed to free it fromalkali and was then dried. The polyethylene obtained had a molecularweight of 1,100,000, as determined viscosimetrically.

760 grams of the high molecular weight polyethylene thus prepared andhaving an average particle size of loop. was placed into a die of 20 cm.in width and 20 cm. in length. The mass was cold-pressed at a specificpressure of 100 kg./sq. cm. and at room temperature whereby blanks of 20x 20 x 2.8 cm. were obtained.

20 of these blanks, in rows of five, were placed side by side into a dieof 100 x 80 cm. The die was placed into a press equiped with heatablepressing plates and subjected to a specific pressure of kg./sq. cm. for6 hours at a temperature of 150 C. Upon cooling, a sheet of plasticizedpolyethylene having a weight of about 1.5 kg. was obtained. This sheetwas completely homogeneous. After sintering of the sheet, the strengthmeasured at the seams between the original blanks was the same as thatof the adjacent portions.

By another embodiment, sheets of 100 x 80 cm. were produced by heatingthe blanks obtained by cold-pressing, in the first compression stage to160 C. in an oven under a protective gas atmosphere; then the heatedslabs were placed side by side into the preheated die of 100 X 80 cm.and compressed in a press which was not heated, or only slightly heated.Here again, sheets having the same good properties were obtained.

Example 2 Polyethylene as prepared in Example 1, i.e. having a molecularweight of 1,100,000 as determined viscosimetrically, was used in thisexample, too. 38 kg. of this material, in a die of 50 cm. in width and80 cm. in length, were cold-pressed at a specific pressure of 130 kg./sq. cm. This resulted in blanks of 14.2 cm. in thickness.

The cold-pressed blanks were compressed into sheets in a heatable pressfor about 8 hours and at a temperature of 160 C. The sheets, whfie hot,were subsequently placed one upon another and compressed for aboutminutes under a specific pressure of 18 kg./ sq. cm. with no additionalsupply of heat. A block of polyethylene was obtained having a weight of152 kg. and a size of 50 x 80 x 32 The block was completely homogeneousand showed no reduction in strength at the seams.

It should be understood that the process can be carried outwith equalsuccess when the laminates are subjected in the second compression stageto pressures of 10-25 kg./ sq. cm. at temperatures ranging from 150 to180 C.

What I claim is:

1. A process for the production of shaped, filled-out solid blocks ofplasticized high-density polyethylene in a plurality of compressionstages wherein in a first compression stage shape-retaining blanks areproduced from finely divided high-density polyethylene whichsubsequently are plasticized in a second compression stage underapplication of heat, which comprises compressing said finely dividedhigh-density polyethylene substantially at room-temperature at pressuresranging from 100 to 180 kg./cm. to form said blanks, placing at leasttWo of said blanks in a face-to-face position, and compressing the sameinto an integral and seamless unit by subjecting them to temperatures of150-180" C. and to pressures of 10 to 25 kg./cm.

2. The process as defined in claim 1, wherein the pressure in said firststage is at least five times that used in said second stage.

3. The process according to claim 1 wherein the blanks obtained in thefirst stage consist of rectangles having 6 surfaces located at rightangles with respect to one another.

4. The process according to claim 1, wherein the blanks obtained in thefirst stage consist of circular disks with parallel end faces.

5. The process according to claim 1, wherein the shaped blanks of thefirst compression stage are arranged side by side in one layer, beforethey are subjected to further compression.

6. The process according to claim 5, wherein the shaped blanks of thefirst compression stage are arranged in superposed position before theyare subjected to further compression.

7. The process according to claim 1, wherein blanks having the samechemical composition and the same physical properties are used.

8. The process according to claim 1, wherein blanks of difierentmolecular weight are combined to produce an integral shaped body.

References Cited in the file of this patent UNITED STATES PATENTS2,428,977 Mares Oct. 14, 1947 2,440,190 Alfthan Apr. 20, 1948 2,597,704Carlson May 20, 1952 2,736,925 Heisler et al. Mar. 6, 1956 2,800,945Schilling July 30, 1957 2,824,090 Edwards et al Feb. 18, 1958 FOREIGNPATENTS 610,562 Great Britain Oct. 18, 1948 735,162 Great Britain Aug.17, 1955 OTHER REFERENCES Polyethylene Grabs the Spotlight, pages -92,210 ff., pages 89 and 90 cited, appearing in Modern Plastics, September1955.

1. A PROCESS FOR THE PRODUCTION OF SHAPED, FILLED-OUT SOLID BLOCKS OFPLASTICIZED HIGH-DENSITY POLYETHYLENE IN A PLURALITY OF COMPRESSIONSTAGES WHEREIN IN A FIRST COMPRESSION STAGE SHAPE-RETAINING BLANKS AREPRODUCED FROM FINELY DIVIDIED HIGH-DENSITY POLYETHYLENE WHICHSUBSEQUENTLY ARE PLASTICIZED IN A SECOND COMPRESSION STAGE UNDERAPPLICATION OF HEAT, WHICH COMPRISES COMPRESSING SAID FINELY DIVIDEDHIGH-DENSITY POLYETHYLENE SUBSTANTIALLY AT ROOM-TEMPERATURE AT PRESSURESRANGING FROM 100 TO 180 KG./CM.2 TO FORM SAID BLANKS, PLACING AT LEASTTWO OF SAID BLANKS IN A FACE-TO-FACE POSITION, AND COMPRESSING THE SAMEINTO AN INTEGRAL AND SEAMLESS UNIT BY SUBJECTING THEM TO TEMPERATUES OF150-180*C. AND TO PRESSURES OF 10 TO 25 KG./CM.2.